In northern Alaska, along the banks of the Colville River, a series of fossil bonebeds preserve remnants of the Late Cretaceous world. These ancient environments were quite different from those found farther south.
Even though the climate of Cretaceous Alaska was warmer than that of today, areas near the Colville River deposits were cold enough to support permafrost and ice fields. This area was not a tropical paradise, but a cooler environment where the average annual temperature was only about 41 degrees Fahrenheit. Nevertheless, the multiple fossil sites show that this place was home to a wide variety of dinosaurs including tyrannosaurs, ceratopsians, hadrosaurs and pachycephalosaurs.
The discovery of such rich assemblages of polar dinosaurs is relatively new. The fact that there were dinosaur fossils in northern Alaska became known only in the 1980s, and excavations since that time have found multiple bonebeds along a 27-mile stretch of the Colville. The diversity of dinosaurs and the number of rich fossil sites was not expected, but what could have caused the formation of so many bonebeds so close to each other?
According to a Palaeogeography, Palaeoclimatology, Palaeoecology paper published this year by Anthony Fiorillo, Paul McCarthy and Peter Flaig, the answer might be found in the colder areas that bordered the environments represented by the bonebeds. As reconstructed by the paleontologists, the dinosaur-bearing deposits are indicative of a relatively warm coastal plain. Right next door, however, the Brooks Range mountains were being pushed up, creating a colder environment where permafrost and snow regularly formed during the coldest part of the year. When summer returned, the ice and snow of the Brooks Range melted, sending floods down into the coastal area where the dinosaurs dwelt.
The jumbles of bones along the Colville are consistent with this scenario. Paleontologists working these sites are not finding beautifully-articulated skeletons, but are instead discovering many bones thrown together, some of which are associated (that is, came from the same animal even though they have fallen out of their natural placement). The bones don't show signs of cracking that would indicate that they were lying exposed on the ground for long periods of time before being buried, nor do they show signs of damage caused by being transported by water over long distances.
Based on the geology of the area and the details of the bones, the bonebeds along the Colville appear to have been created by intense, seasonal floods that quickly killed and buried dinosaurs living on the coastal plain. As the authors themselves state, "The Cretaceous arctic of northern Alaska may have witnessed the coastal plain being a seasonal killing field."
Curiously, however, the way the bonebeds were created caused some dinosaurs to become preserved more often than others. Juvenile dinosaurs appear to have been especially vulnerable. Young dinosaurs are quite common in these assemblages, and this may indicate that the seasonal floods struck during a time of year when young dinosaurs were particularly abundant. This is an important fact. Rather than migrating in during the warmest months, young dinosaurs were probably born and grew up in this place, meaning that dinosaurs were probably living hear year-round. Though we often think of dinosaurs as inhabitants of warm, tropical environments, these polar dinosaurs almost certainly experienced snow. Imagine: herds of hadrosaurs trudging across the plain as snow falls, while a tyrannosaur stalks them from a distance. The dinosaurs did not just barely hang on in this cold place; they thrived there.
And with visions of snowbound dinosaurs dancing in our heads, all of us here at Dinosaur Tracking want to wish you happy holidays!
Fiorillo, A., McCarthy, P., & Flaig, P. (2010). Taphonomic and sedimentologic interpretations of the dinosaur-bearing Upper Cretaceous Strata of the Prince Creek Formation, Northern Alaska: Insights from an ancient high-latitude terrestrial ecosystem Palaeogeography, Palaeoclimatology, Palaeoecology, 295 (3-4), 376-388 DOI: 10.1016/j.palaeo.2010.02.029